The present disclosure relates to superconducting circuits. More particularly, the disclosure relates to systems and methods for quantum information processing and quantum computation.
In the field of quantum computation, the performance of quantum bits (“qubits”) has advanced rapidly in recent years, with preliminary multi-qubit implementations leading toward surface code architectures. In contrast to classical computational methods that rely on binary data stored in the form of definite on/off states, or bits, quantum computation takes advantage of the quantum mechanical nature of quantum systems. Specifically, quantum systems are described using a probabilistic approach, whereby each system includes quantized energy levels whose state may be represented using a superposition of multiple quantum states.
Superconductor-based qubits represent good candidates for quantum computation because of the low dissipation inherent in superconducting materials, and the long coherence times required for performing useful operations. In addition, superconducting circuits can be micro-fabricated using conventional integrated-circuit processing techniques, which, in principle, allows for a large number of superconducting qubits to be produced. However, scaling up from a few qubits to a large-scale qubit architecture present a number of technical challenges. Specifically, quantum measurement and control becomes increasingly more difficult, and requires additional resources, infrastructure and complexity.
Transmon qubits, in particular, have been largely responsible for the significant breakthrough in superconducting quantum information processing over the last decade. The transmon qubit is a modification of the traditional Cooper-pair box (CPB) qubit, which is formed using a superconducting island connected to a grounded reservoir via a Josephson junction. Cooper pairs can tunnel onto the superconducting island, and charge being adjustable by a gate capacitively coupled to the island. However, the islands are exposed to random electric fields from fluctuating charged impurities, which leads to charge noise that affects coherence of the qubit. To address this issue, the transmon qubit includes a large shunting capacitance in addition to CPB qubit structure. As a result, the transmon qubit has been shown to be capable of achieving long coherence times, high-fidelity gates, and reliable readout.
However, the weak anharmonicity of the transmon qubit presents a substantial challenge in pushing the fidelities higher. Fundamentally, the main issue is that both qubit memory and qubit interaction is obtained using transitions with nearly identical frequencies and matrix elements. This prevents the decoupling of the qubits from their dissipative environment, e.g., due to the dielectric loss, without proportionally increasing the gate time. Moreover, in larger arrays, it becomes harder to address individual qubits without affecting other qubits. This is because stronger coupling between transmon qubits requires a smaller detuning of their frequencies, which in turn enhances the uncontrolled state leakage outside of the computational subspace.
In atomic systems, qubit states are chosen in such a way that the transition between them is forbidden by the selection rules to provide long coherence in the computational subspace. Quantum gates and qubit readout are performed through transitions outside of that sub-space with stronger coupling to electromagnetic fields. Such separation of quantum states for information storage and processing allows one to perform many high-fidelity gates before the qubit state is spoiled by decoherence. This was realized in architectures based on nitrogen-vacancy centers, trapped ions, and Rydberg atoms. In superconducting systems, the idea of separating information storage and processing has led to experiments in which the qubit quantum state is stored in a high-quality microwave resonator (e.g. as a single photon or a multi-photon state), while the physical superconducting qubits are used only for short times during gate realizations.
Given the above, there is a need for systems and methods for quantum computation that are scalable and capable of achieving a high degree of fidelity and control.